Re: [PATCH] io_uring: consistently use rcu semantics with sqpoll thread

[Keith Busch <kbusch@kernel.org>]

On Tue, Jun 10, 2025 at 02:04:41PM -0600, Jens Axboe wrote:
> On 6/10/25 1:30 PM, Keith Busch wrote:
> > From: Keith Busch <kbusch@kernel.org>
> > 
> > It is already dereferenced with rcu read protection, so it needs to be
> > annotated as such, and consistently use rcu helpers for access and
> > assignment.
> 
> There are some bits in io_uring.c that access it, which probably need
> some attention too I think. One of them a bit trickier.

Oh, sure is. I just ran 'make C=1' on the originally affected files, but
should have ran it on all of io_uring/.

I think the below should clear up the new warnings. I think it's safe to
hold the rcu read lock for the tricky one as io_wq_cancel_cb() doesn't
appear to make any blocking calls.

---
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index cf759c172083c..c6502197eb6b2 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2906,10 +2906,12 @@ static __cold void io_ring_exit_work(struct work_struct *work)
 			struct task_struct *tsk;
 
 			io_sq_thread_park(sqd);
-			tsk = sqd->thread;
+			rcu_read_lock();
+			tsk = rcu_dereference(sqd->thread);
 			if (tsk && tsk->io_uring && tsk->io_uring->io_wq)
 				io_wq_cancel_cb(tsk->io_uring->io_wq,
 						io_cancel_ctx_cb, ctx, true);
+			rcu_read_unlock();
 			io_sq_thread_unpark(sqd);
 		}
 
@@ -3142,7 +3144,7 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
 	s64 inflight;
 	DEFINE_WAIT(wait);
 
-	WARN_ON_ONCE(sqd && sqd->thread != current);
+	WARN_ON_ONCE(sqd && rcu_access_pointer(sqd->thread) != current);
 
 	if (!current->io_uring)
 		return;
--